DESCRIPTION (Applicant's Abstract): This proposal will study the embryonic and postembryonic growth of the mushroom bodies, a learning and memory center in insects that shares many properties with mammalian hippocampus. Commonalties include structural organization, context-dependent olfactory and visual memory, adaptive sensory association, and molecular pathways involved in memory formation. The specific aims of this project are directed at investigating the formation of synaptic and chemical modules that divide the mushroom body into parallel ensembles of intrinsic fibers that provide intermediate circuits between afferent and efferent neurons. The efferent neurons in the adult system have been shown to change their electrophysiological response properties during sensory association, learning, and memory. Dynamic interactions that underlie the development of sensory systems, and which occur only during a specific developmental window, may be employed throughout life in neural centers that underlie behavioral plasticity. An evolutionarily basal insect, Periplaneta, provides an accessible model system that allows investigation of the development of a memory center by intracellular electrophysiology of neurons developing within synaptic networks that change incrementally over time. These changes may be under the control of steroid hormones, as is the induction of granule cell proliferation in adult hippocampus. In the mushroom bodies, changes involve the formation and differentiation of new laminar subunits by intrinsic neurons or "Kenyon cells" (granule cell equivalents), new sprouting of efferent neuron dendrites, and establishment of new synapses. While the immediate goal of this proposal is to explore the development of this system, a long-term goal is to identify and elucidate similar events in adult structures that learn. The proposed research will provide a significant contribution to understanding the development and functional organization of an accessible and well-defined memory center.